PROJECT SUMMARY/ABSTRACT The brainstem is a complex and early-developing brain region that is responsible for sensory, motor, autonomic, and critical-for-life functions. The first biology-based hypothesis of autism spectrum disorder (ASD) suggested that the reticular formation of the brainstem may be a root cause of ASD symptoms. However, technological barriers have prevented the field from being able to reliably characterize substructures of the brainstem in vivo in children. Excitingly, new technological advances now allow us to examine the microstructural properties of the brainstem's nuclei and individual white matter tracts. The overall scientific premise of this proposal is that brainstem substructures may hold key insights into overall brain development and into the underpinnings of ASD. The overall objective of the proposed work is to identify specific white matter tracts and nuclei within the brainstem that subserve the comorbid sensorimotor and core symptom challenges of ASD and to contextualize brainstem properties in reference to other brain regions implicated in ASD. Given the functions of the brainstem, we hypothesize that the microstructural properties of the brainstem substructures are associated with comorbid sensorimotor symptoms and core social-communication and repetitive-behavior symptoms implicated in ASD. We further hypothesize that the brainstem relates distinctly to other brain regions in ASD, due to the brainstem's role in early brain development. Guided by strong preliminary data, these hypotheses will be tested through three specific aims: 1) Determine extent to which the microstructure of brainstem substructures is associated with individual differences in comorbid sensorimotor symptoms; 2) Determine extent to which microstructure of brainstem substructures is associated with individual differences in core social-communication and repetitive- behavior symptoms; and 3) Identify the distinct correspondence among brainstem substructures and the surrounding brain. A key innovation is that we will accomplish these aims by applying a diffusion-weighted imaging (DWI) technique that addresses the previous challenges of brainstem imaging to provide a clear and anatomically precise image of the brainstem and its substructures. With this technique, we will quantify the microstructure of brainstem substructures (and surrounding brain) in 80 children with ASD (6-9 years old) and 80 age-matched children with typical development. Standardized assessments will characterize sensorimotor and core symptom profiles. The successful completion of this research will provide a quantitative characterization of brainstem substructures in relation to the comorbid sensorimotor features and core symptoms in ASD, and it will provide a quantitative characterization of brainstem substructures in ASD in relation to overall brain metrics. These contributions will be significant because they will advance the understanding of the neurobiological basis for ASD, elucidate the neurological underpinnings of the comorbid sensorimotor challenges and core symptoms in ASD, and provide quantitative biomarkers to be used as outcome measures in clinical trials.